1. Field of the Invention
Projection Imaging.
2. Description of Related Art
In VLSI lithography, poor overlay between the mask levels is one of the main masons production lines produce defective product. See, P. Burggraaf, "Overlay:Lithography's Big Challenge", Semiconductor International, pp. 58-61, (Feb. 1991), and G. Potenza, "Registration Accuracy in Submicron Devices", Microelectronic Engineering, 17 pp. 59-88, (1992). Often product yield could be increased if more time were spent in alignment, but that would decrease product throughput. A good production line tries to find the most profitable compromise between alignment time and throughput.
In step-and-repeat VLSI production, there are two time consuming steps: the global fine alignment, in which deviations of alignment marks from their ideal locations on the wafer are mapped out before exposure begins; and in stepping from one exposure site to the next, under precise interferometer guidance. When the wafer is stepped to a new site for either alignment or exposure, additional time is consumed waiting for vibrations to dampen out. The time spent aligning and moving from site to site is ordinarily greater than the time spent exposing the wafer.
It would seem that an ideal alignment strategy would be to align at each site just before exposure. However, there may be 50 or more exposure sites on a wafer, and alignment time would be excessive. The normal practice is to move the wafer stage to a number of sites, perhaps 9 or 16, and measure the coordinates of the alignment marks to accommodate wafer distortion, possibly created during a preceding processing step. The computer then maps the wafer, determining where the other alignment marks are expected to be by interpolation. Under interferometer guidance, the wafer is then moved to the first exposure site, the chip is exposed, and then, by dead reckoning and not doing any more alignment, moved to subsequent exposure sites. At present, this strategy is the best compromise between high yield and high throughput.
Scanning systems have similar problems. First, there is a global fine alignment, and then a scan in which both mask and wafer move continuously, both guided by interferometers. The interferometers must keep both the mask and wafer stages in fine alignment during the scan. This often puts restrictions on scan speed due to interferometer limitations in data handling.
Often overlay between levels can be improved with more alignment time. But time spent aligning to improve the yield comes at the expense of total product output. A good production line makes the most profitable compromise between alignment time and throughput.
A much more attractive system would be one in which the exposure and the alignment would be done at the same position, thereby avoiding the step between alignment and exposure. This may be done with a through-the-lens system, in which the alignment pattern is on the lithographic mask and the lithographic projection lens is used to image the mask alignment mark on the wafer alignment mark (or vice versa). This requires focusing the alignment pattern on the wafer and, because of chromatic aberrations, ordinarily requires use of the same light wavelength used in exposure. An unwieldy alternative inserts compensating optics, and avoids resist exposure by using a longer wavelength for alignment.
Several commercial systems use two-position through-the-lens alignment--aligning at a first position at a first wavelength and exposing at a second position with another wavelength. See, SPIE, vol. 1264, pp. 534-547 (1990). While assuring accurate sensing at the first position, accuracy at the critical moment is dependent on tracking accuracy--and this with additional expenditure of time.